1. Field of the Invention
The invention relates to a method for forming a dopant profile proceeding from a surface of a plate-shaped or wafer-shaped silicon-based semiconductor device made of silicon with phosphorus as a dopant, by applying a phosphorus dopant source onto the surface, forming a first dopant profile with the dopant source present on the surface, removing the dopant source and forming a second dopant profile having a greater depth in comparison to the first dopant profile.
2. Description of Related Art
For the production of a highly doped surface region of a semiconductor device comprising p-silicon, in order to form an emitter in a solar cell and simultaneously to make possible a good contacting, according to the prior art, a phosphorus source can be applied in order to propel phosphorus from this source into the semiconductor. In this case, with a high dopant concentration of phosphorus, SixPy precipitates are usually formed in the silicon surface. The high dopant concentrations, however, according to general opinion, are necessary in order to produce emitters that are sufficiently deep (≧0.2 μm deep) in methods that are suitable for the mass production of silicon solar cells, emitters that simultaneously have a sufficient surface concentration of phosphorus, which permits the use of Ag pastes for forming contacts. Usually, the dopant is driven in during a temperature (T)-time (t) treatment step.
Advantageously, a method can be used in which the dopant is driven in during two T-t treatment steps, the diffusion source optionally being removed after the first T-t step (WO-A-2010/066626).
Removal of the dopant source after the first diffusion step means that dopant that is still present as well as phosphosilicate glass forming during the temperature treatment are removed.
In the course of the typical diffusion method, a phase of the composition SixPy or Six, PyOz, is produced which can crystallize out in the form of orientation-dependent needles (see FIG. 1) in the highly doped Si layer in the vicinity of the surface. These precipitates have a very small conductivity, can lead to the recombination of charge carriers, produce crystal lattice disruptions in the Si crystal, and can lead to mechanical stresses in the Si surface, which can adversely affect the electrical properties of the semiconductor device.
In order to be able to produce solar cells with lower surface concentrations for highly efficient designs on thin wafers, it is important to use a cost-effective process, such as an in-line process with liquid phosphorus sources as the dopant source for the diffusion step, which does not produce precipitates in the surface of the wafer.
Phases of the composition SixPy or Six, PyOz or the precipitates cannot be removed by HF solutions, as are usually employed for removing the phosphosilicate glass layer after diffusion.